The present invention generally relates to a thermal insulator and more particularly, to a composite thermal insulator utilizing evacuated powder insulation.
Conventionally, for thermal insulators, there have been known inorganic materials such as glass wool, asbestos, ceramic foam, calcium silicate, etc., and organic materials represented by expandable substances such as polystyrene, epoxy, polyurethane or the like, and these materials are employed for various applications such as heat insulating properties, heat resistance, mechanical strength, workability, economical aspect, etc.
Meanwhile, as low temperature heat insulators for refrigerators and the like, expandable materials such as polyethylene foam, expanded polystyrene, foam rubber, rigid polyurethane foam, phenol foam, etc. have been mainly employed, with heat conductivities in the range of 0.015 to 0.037 kcal/mh.degree.C., but from the standpoint of energy conservation, insulating materials having still more favorable heat insulating properties have been required.
On the other hand, for cryogenic or ultra-low temperature heat insulating materials used, for example, for a liquid nitrogen tank or the like, expanded perlite powder subjected to evacuation to form a high vacuum lower than 0.01 Torr has been used, but in this case, a container in which the expanded perlite is filled must be made of a thick metallic material to withstand the high vacuum, and such a requirement presents an obstacle in the applications of the evacuated powder insulation to various commodities.
A thermal insulator constituted by filling a heat insulating material in a plastic container for subsequent evacuation to form a vacuum, has a heat conductivity less than 0.01 kcal/mh.degree.C. to show a favorable heat insulating property, but since plastics are generally provided with a large air transmittance as compared with metallic materials, the heat insulating property thereof deteriorates with time. For reducing the air transmittance of plastics as described above, there has been conventionally proposed one method in which the plastic container is coated with an expanded resin, such as expanded polyurethane and the like, and a composite thermal insulator having such a construction is fully described, for example, in Japanese Laid Open Patent Specification Tokkaisho No. 57-96852. However, even in this prior art thermal insulator, there is still such a drawback that part of Freon (name used in trade and manufactured by Du Pont of the U.S.A.) gas used for expansion enters the interior of the plastic container, thus resulting in deterioration of the heat insulating property of the composite thermal insulator.